power: bcl_peripheral: Update the ADC to vbat conversion formula

Update the ADC value to battery voltage conversion formula
to accommodate the 2 LSB ADC value error. Without this
correction, the existing formula was estimating a lower
battery voltage by 2 LSB ADC value.

CRs-Fixed: 919926
Change-Id: Ibfffbbb8923504c30bdc4032db27f19d0a00e808
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>